In recent years, with the rising awareness of global environmental protection, improvements in fuel efficiency have been strongly required for reducing automobile CO2 emissions. This has led to active attempts to achieve gauge reduction by increasing the strength of automobile body materials to reduce automobile weight. However, the increase in strength of steel sheets may possibly cause the reduction in ductility thereof. Therefore, the development of a high-strength, high-ductility steel sheet is demanded.
In order to increase the strength of steel sheets, a solid solution strengthening element such as Si, Mn, P, or Al is added. In particular, Si and Al have an advantage that the strength of steel can be increased without impairing the ductility thereof; hence, a Si-containing steel sheet is promising as a high-strength steel sheet. However, in the case of manufacturing a galvanized steel sheet or a galvannealed steel sheet using a high-strength steel sheet containing a large amount of Si as a base material, there are problems below.
In the course of manufacturing a galvanized steel sheet, after heating and annealing are performed at a temperature of about 600° C. to 900° C. in a non-oxidizing atmosphere or a reducing atmosphere, galvanizing is performed. However, Si in steel is an oxidizable element. Therefore, Si is selectively oxidized in a non-oxidizing or reducing atmosphere usually used, is concentrated at the surface, and forms an oxide. Since the oxide reduces the wettability with molten zinc during plating to cause bare spots, the wettability decreases sharply with the increase in concentration of Si in steel and therefore bare spots are frequently caused. Even in the case of causing no bare spots, there is a problem in that the controllability of coating weight is poor or alloying is significantly delayed. In particular, the delay of alloying is likely to cause a difference in alloying rate between in the longitudinal direction and cross direction of a steel sheet; hence, it is difficult to achieve a uniform surface.
Furthermore, it is difficult to remove scale from Si-containing steel by descaling in a hot rolling process and therefore scale defects called red scale are caused in the surface thereof. Even in the case where no red scale is formed, water is unevenly applied to a surface of a steel sheet and therefore regions different in scale removal in the cross direction of the steel sheet are present. While scale remaining after descaling is removed by pickling after hot rolling, the regions different in scale removal have different surface properties. Therefore, unevenness occurs in a subsequent galvannealing step to cause defects with a striped pattern.
Among these problems, as for the surface concentration of Si, Patent Literature 1 proposes a method for improving the wettability with molten zinc in such a way that after iron oxide is formed on a surface of a steel sheet by heating the steel sheet in an oxidizing atmosphere in advance, the steel sheet is subjected to reduction annealing. Patent Literature 2 proposes a method for suppressing the surface concentration of Si in such a way that the potential of oxygen is reduced in a reduction annealing process.
On the other hand, as for descaling, Patent Literature 3 proposes a method for enhancing descaling by increasing the spraying pressure of high-pressure water.